As the high functionality of small portable devices such as mobile telephones or notebook personal computers progresses, increases in the energy density of secondary batteries used as the power supplies thereof are anticipated. A non-aqueous solvent-type lithium secondary battery using a carbonaceous material as an anode has been proposed as a secondary battery having a high energy density (Patent Document 1).
In recent years, large secondary batteries, having high energy density and excellent output characteristics, are being mounted in electric vehicles in response to increasing concern over environmental issues. For example, increasing use of non-aqueous electrolyte secondary batteries is anticipated in vehicle applications such as in electric vehicles (EV), which are driven solely by motors, and plug-in hybrid electric vehicles (PHEV) and hybrid electric vehicles (HEV) in which internal combustion engines and motors are combined. In particular, lithium-ion secondary batteries, which are non-aqueous solvent-type lithium secondary batteries, are widely used as secondary batteries having high energy density, and further increases in energy density are anticipated in order to extend the cruising distance with one charge in EV applications.
High energy density requires a large doping and de-doping capacity of lithium in the anode material, but the theoretical lithium storage capacity of graphitic materials that are mainly used presently is 372 Ah/kg, and there are theoretical limits. Further, when forming an electrode using a graphitic material, a graphite intercalation compound is formed as the graphitic material is doped with lithium, which increases the interlayer spacing. The interlayer spacing returns to normal as a result of de-doping the lithium doped between layers. Therefore, with a graphitic material having an advanced graphite structure, repeated doping and de-doping of lithium (repeated charging and discharging in the secondary battery) causes a repeated increase and return of spacing, which tends to lead to the breakdown of graphite crystals. Accordingly, secondary batteries formed using graphite or a graphitic material having an advanced graphite structure are said to have poor charging and discharging repeating characteristics. Further, in batteries having such an advanced graphite structure, a problem in which the electrolyte solution tends to degrade easily at the time of battery operation has also been indicated.
On the other hand, alloy-based anode materials containing tin, silicon, or the like have also been proposed as materials having a large capacity, but the durability is insufficient, so the use of such materials is limited.
In contrast, non-graphitic carbon materials have excellent durability and have a large capacity exceeding the theoretical lithium storage capacity per unit weight of the graphitic material, so various proposals have been made for such materials as high-capacity anode materials. For example, the use of a carbonaceous material obtained by subjecting a phenol resin to heat treatment as an anode material for a secondary battery has been proposed (Patent Document 2). However, when an anode is produced using a carbonaceous material obtained by subjecting phenol resin to heat treatment at a high temperature such as 1900° C. or higher, for example, there is a problem in that the doping and de-doping capacity of the active material such as lithium into the anode carbon is small. In addition, if an anode is produced using a carbonaceous material prepared by heat-treating a phenol resin at a relatively low temperature, such as around 480 to 700° C., the doping amount of lithium used as an active material is large, which is preferable. However, lithium doped into an anode carbon may not be completely de-doped and a large amount of lithium may remain in the anode carbon. Such a wasteful consumption of lithium, which is an active material, is a problem.
In addition, in the production process of a carbonaceous material, a production method for carbon for a lithium secondary battery has been proposed, the production method comprising the steps of: obtaining halogenated dry-distilled carbon by bringing a halogen-containing gas into contact with dry-distilled carbon; a de-halogenating step of obtaining a de-halogenated carbon by removing some or all of the halogens in the halogenated dry-distilled carbon; and a pore preparation step of bringing the de-halogenated carbon into contact with pyrolytic hydrocarbon (Patent Document 3). With this method, a large doping and de-doping capacity can be achieved. However, lithium doped into an anode carbon may not be completely de-doped and a large amount of lithium may remain in the anode carbon. Such a wasteful consumption of lithium, which is an active material, is a problem.